1. Field of the Invention
The present invention relates to overcurrent protection of semiconductor devices for various uses, particularly, for use in a driving circuit for driving a uni-phase or tri-phase motor.
2. Description of the Background Art
FIG. 1 shows a conventional driving circuit for driving a tri-phase motor. In the driving circuit, IGBTs (insulated gate bipolar transistors: hereinafter referred to simply as transistors) Q1 to Q6 serve as switching devices. The transistors are in pairs; a pair Q1-Q2, a pair Q3-Q4 and a pair Q5-Q6, and the transistors in each pair are connected in a standard totem pole relation. The three transistor pairs are bridge-connected between power supplies P and N in a tri-phase configuration. Junctions of the transistors are connected to input terminals of a motor M.
Free wheeling diodes (hereinafter referred to merely as diodes) D1 to D6 are connected reverse-parallel to the transistors Q1 to Q6, respectively. Driving circuits Dr1 to Dr6 are provided for driving and protecting the transistors Q1 to Q6, and give their outputs to the gates of the transistors Q1 to Q6. The driving circuits Dr1 to Dr6, responding to signals received at their input terminals IN1 to IN6, turn on and off the transistors Q1 to Q6 so that operation of the motor M is regulated.
To protect the transistors Q1 to Q6 against an overcurrent developed therein, current transformers CT1 to CT6 and overcurrent detection circuits S1 to S6 are disposed, respectively. The current transformers CT1 to CT6 independently detect forward currents flowing in the transistors Q1 to Q6. Each overcurrent detection circuit finds whether a current in its associated current transformer exceeds a predetermined threshold level. Detecting an overcurrent, the overcurrent detection circuits S1 to S6 generate an inactivate signal which is given to inactivation control terminals B1 to B6 of the driving circuits Dr1 to Dr6, respectively. The overcurrent detection circuits S1 to S6 thus provide feedback, which in turn interrupt the currents to the transistors Q1 to Q6 (i.e., causing turn-off of the transistors).
Overcurrent protection is otherwise achieved by detecting a net overcurrent across the tri-phase circuit in bridge configuration as a whole by a current transformer CT7 for detecting a current on a bus line and an overcurrent detection circuit S7, or alternately, by current transformers CT8 to CT10 for detecting an output current and an overcurrent detection circuit S8, and by thereafter providing feedback simultaneously to the driving circuits Dr1 to Dr6.
These approaches, although being capable of dealing with an overcurrent in the transistors Q1 to Q6 due to a regenerative current from the motor M, are not satisfactory in that an overcurrent across the diode D1, D2 . . . D6 cannot be separately detected. Thus, the diodes D1 to D6 are vulnarable to an overcurrent and accompanying generation of heat, which destroys the diodes D1 to D6.
This is obviated by replacing the diodes D1 to D6 with other types of elements having a larger current capacitance than the switching devices (the transistors Q1 to Q6), but only at the expense of reduction in device size.